Disorders of systemic water balance (i.e., hypo- and hypernatremia) are extremely common among the elderly, chronically ill, and hospitalized patients. Hyponatremia, particularly if severe, is associated with markedly increased mortality, even in the absence of evident comorbidity. The cation channel TRPV4 is believed to be the long-sought central sensor of systemic hypotonicity: it is activated by relative water excess (i.e., hyponatremia/hypotonicity);it is expressed in the osmoregulatory nuclei of the brain;and mice deleted for the gene exhibit abnormal water metabolism. We have identified two non-synonymous single nucleotide polymorphisms in the human TRPV4 gene (i.e., resulting in an amino acid change). When expressed in cultured cells, TRPV4 protein encoded by either of these minor alleles is less responsive to hypotonicity than is the wild-type channel. Furthermore, we show in preliminary fashion that one minor allele is significantly over-represented among human subjects with aberrant water metabolism. In 209 healthy elderly subjects, this TRPV4 allele was approximately five-fold more prevalent among subjects with a serum sodium concentration of <135 mEq/liter than in normonatremic subjects. We propose to investigate this functionally significant TRPV4 minor allele on two levels. In Aim I, we will phase, in preliminary fashion, the haplotype(s) that includes this minor allele of interest, to exclude the possibility that a tightly linked polymorphism contributes to the hyponatremic phenotype. Any newly identified polymorphisms in the haplotype will be tested for functional significance using in vitro assays addressing transcription, mRNA stability and splicing, and protein function. In Aim II, we will test for an association between the TRPV4 minor allele and the presence of clinical hyponatremia in a second, much larger, data set. Banked serum and DNA from 6000 healthy elderly subjects in the Osteoporotic Fractures in Men (MrOS) study will be analyzed for serum sodium concentration (as an index of water balance) and TRPV4 genotype, respectively. With this combined approach, we will address the physiological significance of the TRPV4 alleles at the cellular level, and at the human population level. The over-all objective is to prospectively identify patients at increased risk for potentially life-threatening retention of free water, for example when treated with hypotonic fluids in the post-operative setting, or when treated with a thiazide-type diuretic. Many chronic disease states and several widely prescribed medications can cause dangerous abnormalities in water balance in an unpredictable fashion;perturbed water balance can generally only be detected through measurement of the serum sodium concentration . This proposal aims to increase our understanding of disorders of water balance among the elderly, and to genetically identify patients who may be at increased risk for potentially life-threatening retention of water.